Method for electrical discharge machining employing periodic extended pulse off time

ABSTRACT

Electrical discharge machining is provided by generating a series of rectangular machining power pulses at a controllable on-off time duration. Provision is made for introducing extended off time duration between groups of pulses to contribute to the improvement of cutting stability. In an alternate embodiment of the invention, a single on time pulse is provided intermediate a pair of spaced pulse groups.

United States Patent 11 1 Verner Apr. 22, 1975 1 1 METHOD FOR ELECTRICALDISCHARGE 3.485.988 12/1969 Scnnowitz 219/69 P MACHINING EMPLOYINGPERIODIC 3.558.842 1/1971 Lirshits.....'. 219/69 P 3.614.368 10/1971Lobur r 219/69 P EXTENDED PULSE OFF TMME 3.697.719 10/1972 Vcmcr et a1219/69 C [75] Inventor: Dalton R. Verner, Orchard Lake.

M' h. Primary Eraminer-Bruce A. Reynolds Asslgneei Coll Industries p tig Attorney. Agent, or Firm-Haukc, Gifford, Patalidis &

Corporation, New York. NY. Dumom [22] Filed: July 19, 1973 [21] Appl.No.: 380,757 [57] ABSTRACT Related US. Application Data t [62] DivisionofScr. No. 228 661 Feb. 23 1972 Pat, No. Elecmca' d'scharge machmmgpmv'ded by genera" 3789 |82 ing a series of rectangular machining powerpulses at a controllable on-off time duration. Provision is made 1521us. 01 219/69 M; 219/69 P for introducing extended time duration between[51] Int. Cl 823p 1/08 groups 0f Pulses to contribute to the improvementof 58] n of Search H 219/69 M 69 P. 69 C cutting stability. In analternate embodiment of the invention, a single on time pulse isprovided intermedi- [56] Reerences Cited ate a pair of spaced pulsegroups.

UNITED STATE S PATENTS 4 Claims, 8 Drawing Figures 3.439.145 4/1969Scnnow1tz 219/69 P e 50 42 e-e Z6 1,4 1/4 a /5 M 44 4'4 e .2 i 211 ll! 1JUL 1 METHOD FOR ELECTRICAL DISCHARGE MACHINING EMPLOYING PERIODICEXTENDED PULSE OFF TIME REFERENCE TO RELATED APPLICATION The presentapplication is a division of my US. application, Ser. No. 228.661, filedon Feb. 23, 1972. for Method and Apparatus for Electrical DischargeMachining Employing Periodic Extended Pulse Off Time". now issued as US.Pat. No. 3,789,182 patent which is of common ownership herewith.

BACKGROUND OF THE INVENTION The field to which this invention relates isthat known as electrical discharge machining, hereinafter sometimesreferred to as EDM. in which material is removed from an electricallyconductive workpiece by the action of electrical gap discharges passedbetween a tool electrode and the workpiece. A servo feed system isnormally used to provide relative movement to maintain an optimum gapspacing between the electrode and workpiece as the work-piece materialis progressively removed. A dielectric liquid coolant is circulated andrecirculated under pressure through the gap during machining operation.In order to provide machining with reliable and predictable results, anelectrical discharge machining circuit of the independent pulsegenerator type is preferably used to provide machining power pulses ofprecisely controllable frequency and on-off time. In this particulartype of EDM circuit. the pulse generator may be embodied as amultivibrator. square wave oscillator or the like. Under adverse cuttingconditions. such as when poor coolant flow condition is present or whenthe proper coolant flow pattern is particularly difficult to establishby reason of the particular geometry of the electrode and thework-piece, it becomes difficult to maintain stable EDM cutting. Similarproblems are met with certain types of workpiece metals now being usedfor the production of dies, such as. for example. cast iron as it is nowused in large size die production.

BRIEF STATEMENT OF THE INVENTION It has been found advantageous when theabove described conditions are encountered to provide a control for thepulse generator itself such that the machining power pulses areperiodically interrupted by extending the normal pulse off time,preferably for a time duration at least equal to at least two times thenormal pulse off time duration. It appears that machining with theresultant spaced trains of power pulses has the effect of permitting gaprecovery, thus substantially contributing to and improving the stabilityof cutting. Further. with respect to cast iron cutting. this mode ofcutting has the effect of eliminating the undesirable molten pool andspatter effect which sometimes occurs on the workpiece surface.

BRIEF DESCRIPTION OF THE DRAWINGS Reference is made to the appendedspecification which explains the present invention and to the drawingsin which like numerals and letters are used to refer to like elementswhich are shown throughout the several drawings and wherein:

FIG. 1 is a combined schematic and block diagrammatic representation ofan electrical discharge machining power supply incorporating the presentinvention;

FIGS. 2 and 3 represent diagrams of two control voltage waveforms whichare typical outputs of a second pulse generator which may be used inpracticing the present invention;

FIG. 4 is a schematic drawing of one embodiment of the second pulsegenerator to be employed;

FIGS. 50 and 5b are gap current diagrams showing the normal electricaldischarge machining pulse waveform and the pulse waveform available fromthe several embodiments of the invention; and

FIGS. 6 and 7 are schematic diagrams showing an ad ditional embodimentof the present invention in which an electronic counter and controlcircuit are included.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to FIG. 1, themain machining power DC source 10 is shown connected in series with thepower conducting or principal electrodes of an output transistor 12 andin series with the machining gap. which gap includes a tool electrode 14and a workpiece 16. It will be understood that a plurality of outputtransistors 12 may be connected in parallel to provide the requiredmagnitude of cutting current. The gap current level is selectivelycontrolled by the magnitude of a variable series resistor 18. Theelectrical discharge machining power supply further includes a pulsegenerator embodied as a multivibrator stage 26 and one or moreintermediate drive stages 28 which are used to turn the output stagetransistor 12 on and off with precisely controllable pulse duty factorand frequency. In the interest of brevity, the driver stage 28 has beenillustrated in a block diagrammatic form.

The multivibrator 26 includes a pair of transistors 34 and 36 which arebiased and coupled for alternate switching operation in the astablemultivibrator mode. The transistors 34, 36 are each connected through acorresponding load resistor 38, 40 to the negative terminal of a DCsource 42. A pair of banks of crosscoupling capacitors 44 and 46 areconnected to the transistors 34, 36, respectively. The switchingcapacitor elements have been shown as adjustable capacitors in theinterest of simplification. It will be seen that the collectors of thetransistors 34, 36 are each crosscoupled to the opposing transistor basefor the purpose of controlling the multivibrator 26 output pulsefrequency and, accordingly, controlling the frequency of the machiningpower pulses provided to the machining gap. A pair of adjustableresistors 48 and 50 are included in the multivibrator 26 circuit, withthe machining pulse on-off time controlled by the relative setting ofthose resistors.

Also included in FIG. 1 is a second pulse generator 52 which is shown inblock form as it is connected in the circuit across the terminals A andB. A diode 54 is included in series between the terminal A and the baseof the off time" control transistor 36. The output signal across theterminals A and B is shown in FIGS. 2 and 3. The output may be of eithervoltage waveform illustrated in accordance with the particular type ofpulse generator 52 used.

FIG. 4 shows one embodiment of the second pulse generator 52 which maybe used in conjunction with the multivibrator 26. It will be seen toinclude a variac 56 which is connected to a line voltage source across apair of terminals X and Y, with the magnitude of the voltage outputcontrolled by a slider 58. The voltage pulse output is connectedinductively to a winding 60, passes through a diode 62, alimiting'resistor 64, and finally to the output terminal A. It will beunderstood that the second pulse generator alternately may be amultivibrator similar to the multivibrator 26 but operated at differentbut appropriate on-off time and frequency.

FIG. 6 shows the counter and gating stages used in the alternateembodiment for the present invention. It has been found that the bestmachining results are obtained when the machining power pulses areseparated for a time duration equal to at least two normal off timedurations. This condition is illustrated in FIG. b as compared to FIG.5a and in each case the normal machining power pulses are identified bythe numeral 55. As shown in FIG. 5b, there is a spacing provided of atleast two normal off time durations between the last pulse 55 of amachining pulse group and the first pulse 55 of the next followinggroup. As one improvement to the machining method, I have found it isoftentimes advantageous to include between the two successive groups ofpulses a single machining power pulse such indicated in dash line formand identified by the numeral 55a. This intervening pulse appears tohave the advantage of keeping sufficient power in the gap between pulsetrains to maintain a normal downfeed for the servo system. It furtherappears to facilitate restriking by the first pulse of the nextfollowing pulse train. It will be noted that the pulse 55a is of thesame duration as a normal pulse 55.

In order to provide the method of EDM just described and illustrated inFIG. 5b, there is provided a control circuit including a counter stageof four flipflops 68a, 68b, 68c and 68d together with a plurality ofdiode input gates, two of which are shown at 69a and 69b. The left-handgate 69a is designed to provide eight pulses in each machining pulsetrain, while the righthand gate 69b provides a nine pulse train. Fordifferent work-piece-electrode material combinations and for differentcutting conditions, it is advantageous to have the capability ofchanging the number of pulses in each pulse train. Additional gatingstages may be included in the circuit to give this flexibility. I

Now, with more particular reference to the several counter flip-flopstages 68a through 6811, it will be seen that each contains a flip-flopwith a pair of alternately operable transistors Q and Q The flip-flopsare sym metrically designed and include like value collector resistors70 and 72, and like value bias resistors 74 and 76. The input to thecounter stages is provided at the left-hand terminal 0 from themultivibrator 26 as previously shown in FIG. 1. Each flip-flop furtherincludes a pair of signal diodes 78 and 80 connected to the collectorand base of each transistor 0, and a pair of signal diodes 82 and 84connected to the collector and base respectively of each transistor QCoupling capacitors 86 and 88 are connected to each counter asindicated.

Each counter flip-flop stage further includes provision for a resetpulse after the counting period is ended. In each case, the reset pulseis provided at a terminal Z and passed through a series resistor 88 anda diode 90 to the base of each transistor Q2 of each counter flip-flopstage for reset. The manner and the sequence in which the reset pulse isgenerated will be explained and shown in connection with FIG. 7hereinafter.

The circuit of FIG. 7 includes the control network which serves tocoordinate the operation of the multivibrator 26 with that of the gatingand counter stages. It provides suitable control outputs to turn off andturn on the multivibrator 26 at the right times. Finally, after thecounting operation is completed, it provides for a reset of each of theflip-flop counter stages 68a, 68b, 68c and 68d. Included in the circuitof FIG. 7 are an inverter stage 1000, a power amplifier stage b, amonostable stage 102, a transistor turn off stage 104, a flip-flop stage106 and a final monostable stage 108. For clarification, the severalstages 100a, 100b, 102, I04, 106 and 108 are enclosed by dash lineboxes.

With more particular reference to the inverter stage 100a, it will beseen to include a transistor Q A suitable series load resistor 110 isconnected between a 3+ voltage source and the collector of thetransistor 0,. A signal limiting resistor 114 is connected in serieswith the base of the transistor 0,. The connections between the circuitof FIGS. 6 and 7 are indicatedby the three leads 115, 116 and 117.Included in series with the input from the multivibrator 26 at the leadis a diode 118. Diode 120 is operatively connected as shown in serieswith lead 117. A variable resistance comprising a fixed resistor 121 anda potentiometer 124 is connected between the junction of the diodes 118and 120 and the base of the transistor Q Upon occurrence of the firstmultivibrator pulse after an output on lead 117, a signal is providedthrough a diode 132 which triggers monostable stage 102. At the sametime, a signal is provided from the lead 117 to the base of the invertertransistor Q, to turn it on. This provides a grounding signal to thecounter to stop and hold the existing count through the lead 116.

The monostable stage 102 includes a transistor Q and a transistor 0 Apair of collector load resistors 134 and 136 are connected in the mannershown with biasing resistors 138 and 140 connected in the circuit. Atime constant of operation is selectively controllable through theadjustment -of a variable resistor 142, which is series connectedbetween the B+ source and a fixed resistor 144 and through the selectionof one of a plurality of capacitors 146 and 148 which may conform innumber with the plurality of tapped capacitor switches 44, 46 used tocontrol the on-off time of the multivibrator 26. It will be understoodthat the transistor Q of the monostable stage 102 is biased normally on.Upon receipt of a keying signal from the amplifier stage 100b, thetransistor Q; will be turned off and provide a signal to themultivibrator turn off stage 104.

The stage 104 will be seen to include a transistor Q8 having its baseconnected through a signal resistor 150 to the collector of themonostable transistor Q The transistor Q further has its collectorconnected to the 13+ source through a series resistor 152. The signaloutput from the transistor Q is provided through a resistor 154connected in series with a diode 156 to the terminal A of themultivibrator 26 in the FIG. 1 drawing. A pulse output from thetransistor 0,, is thus effective to turn off the transistor 34 of themultivibrator 26. At the same time, a triggering pulse is provided fromthe monostable stage 102 to the flip-flop stage 106, more particularlyto the base of transistor Q The other operating elements of theflip-flop stage 106 include a transistor O & pair of like-value loadresistors 158 and 160 connected in series respectively with thecollectors of the transistors Q and Q and a pair of biasing resistors162 and 164 which are connected to the respective bases of thetransistors Q and Q The pulse input to the flip-flop stage 106 isprovided through the coupling capacitors 166 and 168 with signal diodesI70, 172 and 174, 176 connected in series with the respective collectorsand bases of the transistors Q and Qm. When the monostable transistor Qreturns to its on stage. it permits restart of the multivibrator 26. Inthe conducting state of the transistor Q the multivibrator 26 is againpermitted to operate. When multivibrator 26 starts to operate, one pulseretriggers the monostable stage 102. The monostable stage 102 thenreblocks the multivibrator 26 through the previously described path. Atthe end of the second time constant of operation of the monostable stage102, the flip-flop 104 is reset to its original state. By this action,the monostage stage 108 is triggered.

The monostable stage 108 includes a pair of transistors Q and Q having apair of series load resistors 178 and 180 connected in series with theirrespective collectors. A pair of bias resistors 182 and 184 are alsoconnected in circuit as shown. Capacitor 186 is connected between thelower end of the resistor 184 and the collector of the transistor Q Itwill be seen that the signal output from the flip-flop stage 106 passesthrough a coupling capacitor 188 and a diode 190 to trigger themonostable stage 108. A separate diode 192 is connected in series with aB-voltage as indicated. Responsive to its retriggering, the monostablestage 108 resets the counter stages 68a-68d through lead Z. Upon resetof the counter, the circuit is placed in readiness for another group ofpulses to be generated through the system. The cycle then is repeated inthe manner described above.

DESCRIPTION OF OPERATION In the circuit of FIG. 1, the multivibrator 26provides a pulse output which is suitably amplified and resquared in theintermediate drive stage 28 to render the output transistor 12alternatively conductive and nonconductive and therefore to provide acontinuous series of machining power pulses across the machining gap asbest shown in FIG. 5a. The machining pulse on time is controlled inphase with the conduction of the transistor 34, while the pulse off timeis controlled in phase with the conduction of the transistor 36. It willbe noted that while the present invention utilizes transistors as theelectronic switches throughout the circuit, the invention is notsolimited. With proper redesign of the circuit by one skilled in theart, any electronic switches may be substituted for the transistorsshown. By electronic switch" is meant any electronic control devicehaving more than two electrodes comprising at least two power conductingor principal electrodes acting to control current flow in the powercircuit, the conductivity of the power circuit being controlled by acontrol electrode within the device, whereby the conductivity of thepower circuit is controlled statically or electrically without movementof any mechanical elements within the device itself. Included withinthis definition by way of example but not limitation are electron tubes,transistors, semi-conductor controlled rectifiers, thyratrons andsimilar electronic devices.

As has already been described, there are problems which arise duringunstable cutting conditions in the gap which may in some cases lead toactual gap short circuiting with resulting damage to the electrode tooland the workpiece itself. Systems have been devised which respond to theexistence of gap short circuit con ditions and by one means or anothereither to totally interrupt power to the gap or to control the powercontent of the following machining power pulses. One example of such ashort circuit protection system is shown in my US. Pat. No. 3,697,719,issued on Oct. I0, 1972, for Pulse Narrowing and Cut-Off ProtectionSystem for Electrical Discharge Machining.

I have found that it is possible by employing the EDM method andapparatus which I have provided to avoid most gap short circuitingthrough control of the machining pulse waveform at the pulse generatoritself, namely through control of the multivibrator 26. The problemsalready described with respect to machining cast iron and similarmaterials may be overcome by providing a periodic recovery period topermit deionization of the gap and attendant cooling effect on the gapelements. This is accomplished by using a second pulse generator orpulse source 52 and providing off time duration control with respect tothe first pulse generator, exemplified by the multivibrator 26 ofFIG. 1. It will be seen that in the operation of the multivibrator 26with the on time and the off time being controlled by the conduction ofthe transistor 34 and by the conduction of the transistor 36,respectively, it is possible by superimposing a control pulse of theproper polarity on the off time control transistor 36 to extend the offtime duration, and in this manner effectively provide machining byspaced pulse trains.

l have further found it to be particularly advantageous to provide aspacing which is equal to at least twice the normal on-off time pulseduration to permit the full gap recovery action to take effect. Itshould be noted that the method of EDM according to the presentapplication is one in which the control of machining pulses isadjustably made and continuously carried out throughout the machiningoperation with no power interruption being made to change andsubstantially restore the original operating parameters. In this manner,it is possible to avoid the excessive hunting and cutting time losswhich is inherent in those systems which respond to a gap short circuitand, at a later time, are returned to normal machining operation.

In the operation of the multivibrator 26, the resistor is first adjustedto preset the off time, while the resistor 48 is adjusted to preset theon time. The voltage pulses from the second pulse generator 52, as theyare seen across terminals A and B, are of much lower frequency than themachining power pulses as shown, for example, in the FIG. 5 waveforms.In order to provide a sufficiently long pulse interruption, the timeduration of the control pulses from the second pulse generator 52 are ofa duration in excess of twice the normal pulse repetition rate.

The drawing of FIG. 5b illustrates the resultant spacing betweenmachining power pulses which occurs as a result of the operation of thesecond pulse generator 52. l have further found that, as a matter ofconvenience, symmetrical pulse generators may be used. It is alternatelypossible to use generators across terminals A and B which operate in asomewhat random fashion to provide a similar off time extension andthereby control the machining power pulses. The alternate embodiment ashas been shown and described in relation to FIGS. 6 and 7 provides for asimilar periodic extension of pulse off time with a counter included toprovide for the selection of the number of pulses in each machininggroup. In addition, provision is made to include an adjustable widthpulse between the trains of machining power pulses.

It will thus be seen that lhave provided an improved method forelectrical discharge machining which is novel and represents asubstantial improvement with respect to cutting work-pieces of amaterial and of a configuration often found difficult to cut even by theEDM process.

Having thus described my invention, 1 claim:

1. The method of electrical discharge machining of an electricallyconductive workpiece by providing machining power pulses in groupsthereto across a dielectric filled gap comprising the steps of:

a. providing a series of machining power pulses across the machining gapat predetermined on and off times;

b. predetermining the number of pulses in machining pulse groups; and

c. periodically extending the off time between such groups of saidpulses to provide a predetermined off time which is substantiallygreater than the normal predetermined off times.

2. The combination as set forth in claim 1 wherein a single pulse ofsubstantially normal on time is provided between said groups of pulses.

3. The method of electrical discharge machining of an electricallyconductive workpiece by providing machining power pulses thereto acrossa dielectric filled gap comprising the steps of:

a. predetermining by a counter the number of pulses to be included inmachining power pulse groups;

b. providing machining power pulses of such groups at predeterminedon-off time pattern across the gap; and

c. introducing an extended pulse off time at uniformly spaced intervalsbetween said groups of pulses whereby the stability of machining ofmetals of the cast iron type is improved.

4. The method of electrical discharge machining of an electricallyconductive workpiece by passing machining power pulses across adielectric coolant filled gap comprising the steps of:

a. generating said pulses in a train of pulses having a predeterminedon-off time;

b. predetermining the number of pulses in each such train by a counter;and

c. periodically extending the pulse off time between such trains tointerrupt machining for predetermined periods exceeding at least twotimes the normal pulse off time duration.

1. The method of electrical discharge machining of an electricallyconductive workpiece by providing machining power pulses in groupsthereto across a dielectric filled gap comprising the steps of: a.providing a series of machining power pulses across the machining gap atpredetermined on and off times; b. predetermining the number of pulsesin machining pulse groups; and c. periodically extending the off timebetween such groups of said pulses to provide a predetermined off timewhich is substantially greater than the normal predetermined offtimes.
 1. The method of electrical discharge machining of anelectrically conductive workpiece by providing machining power pulses ingroups thereto across a dielectric filled gap comprising the steps of:a. providing a series of machining power pulses across the machining gapat predetermined on and off times; b. predetermining the number ofpulses in machining pulse groups; and c. periodically extending the offtime between such groups of said pulses to provide a predetermined offtime which is substantially greater than the normal predetermined offtimes.
 2. The combination as set forth in claim 1 wherein a single pulseof substantially normal on time is provided between said groups ofpulses.
 3. The method of electrical discharge machining of anelectrically conductive workpiece by providing machining power pulsesthereto across a dielectric filled gap comprising the steps of: a.predetermining by a counter the number of pulses to be included inmachining power pulse groups; b. providing machining power pulses ofsuch groups at predetermined on-off time pattern across the gap; and c.introducing an extended pulse off time at uniformlY spaced intervalsbetween said groups of pulses whereby the stability of machining ofmetals of the cast iron type is improved.